linux-kernel - Re: [RFC PATCH] bpf: handle 32-bit zext during constant blinding

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Date:   Wed, 21 Aug 2019 14:00:44 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jiong Wang <jiong.wang@...ronome.com>
Cc:     bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org,
        Michael Ellerman <mpe@...erman.id.au>, netdev@...r.kernel.org
Subject: Re: [RFC PATCH] bpf: handle 32-bit zext during constant blinding

Naveen N. Rao wrote:
> Since BPF constant blinding is performed after the verifier pass, there
> are certain ALU32 instructions inserted which don't have a corresponding
> zext instruction inserted after. This is causing a kernel oops on
> powerpc and can be reproduced by running 'test_cgroup_storage' with
> bpf_jit_harden=2.
> 
> Fix this by emitting BPF_ZEXT during constant blinding if
> prog->aux->verifier_zext is set.
> 
> Fixes: a4b1d3c1ddf6cb ("bpf: verifier: insert zero extension according to analysis result")
> Reported-by: Michael Ellerman <mpe@...erman.id.au>
> Signed-off-by: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
> ---
> This approach (the location where zext is being introduced below, in 
> particular) works for powerpc, but I am not entirely sure if this is 
> sufficient for other architectures as well. This is broken on v5.3-rc4.

Alexie, Daniel, Jiong,
Any feedback on this?

- Naveen